This invention relates to internal combustion engines, and more particularly to systems and methods for controlling and estimating cylinder air charge in direct injection internal combustion engines.
As is known in the art, cylinder air charge estimation and control are important in meeting performance requirements of advanced technology engines, such as direct injection spark ignition (DISI) engines. The objective of the air-charge control in lean-burn, spark-ignited engines is to operate an electronic throttle and the exhaust gas recirculation (EGR) valve in a manner so as to provide the desired gas flow to the engine and the desired burnt gas fraction in this flow for NOx reduction. A secondary objective of air-charge control is to provide updated estimates of the in-cylinder conditions, in terms of charge quantity and burnt gas fraction, as inputs to other Engine Management System (EMS) features, such as torque and fueling control. This function is referred to as charge estimation. The conventional approach to the cylinder air-charge control is open-loop whereby the desired mass flow rates through the EGR valve and throttle are calculated as functions of the desired burnt gas fraction in the intake manifold and of the desired gas flow into the cylinder using the orifice equation.
On a typical DISI engine, an electronic throttle (ETC) is used to control the inlet fresh air while the burnt gas is recirculated from the exhaust manifold back into the intake manifold through an exhaust gas recirculation (EGR) valve. Another valve, referred to as the swirl control valve (SCV), or other charge motion device, such as cam profile switching (CPS), is also present in the intake system to effect different swirl ratios and therefore to provide the mixture motion in the cylinder that optimizes the combustion process, especially in the stratified operation.
Difficulties in developing a simple and robust cylinder air charge controller for a DISI engine result from several special characteristics associated with the lean burn and stratified operation. First, when the engine operates lean, the flow through the EGR valve contains unburnt air that has not been consumed in the previous combustion event, and this, together with the intake manifold dynamics, adds difficulties in accurately estimating the in-cylinder burnt gas fraction. Second, accurate air flow estimates via the orifice equation are hard to obtain at high manifold pressures where a DISI engine may frequently operate. Third, the buildup of soot and other deposits in the EGR conduit and intake ports is more severe on DISI engines because of the stratified operation and high flow volume of EGR. The deposits change the valve flow and engine breathing characteristics and make the charge control system very susceptible to aging. Other factors, such as actuator imperfection due to friction and quantization, also contribute to complicating the problem.
As is known in the art, open-loop charge estimation and control have been used for a conventional PFI engine. For an open-loop charge estimation for a DISI engine, the estimates of flows through the throttle and the EGR valve are based on the orifice equation, the flow into the cylinders based on the speed-density equation, and the burnt gas fraction in the intake manifold based on the manifold dynamic model and mass balance of air and burnt gas. More particularly, the standard orifice equation applied to the throttle and EGR valve gives the following estimates for Wthr (i.e., the fresh air flowing through the throttle) and Wegr (i.e., the recirculated gas flowing through the EGR valve):                               W          thr                =                              f            ⁡                          (                                                                    p                    amb                                                                              T                      amb                                                                      ,                                                      P                    i                                                        P                    amb                                                              )                                ⁢                                    u              thr                        ⁡                          (                              θ                thr                            )                                                          (        1        )                                          W          egr                =                              f            ⁡                          (                                                                    p                    exh                                                                              T                      exh                                                                      ,                                                      P                    i                                                        P                    exh                                                              )                                ⁢                                    u              egr                        ⁡                          (                              θ                egr                            )                                                          (        2        )            
where f is a function (fcn) of upstream pressure, upstream temperature, and the pressure ratio across the throttle and EGR valve given by:                                           f            ⁡                          (                              x                ,                y                            )                                =                                                    γ                                  1                  2                                            ⁡                              (                                  2                                      γ                    +                    1                                                  )                                      ⁢            x                                                            if            ⁢                          xe2x80x83                        ⁢            y                    ≤          0.528                                                              f            ⁡                          (                              x                ,                y                            )                                =                                    xy                              1                γ                                      ⁢                          {                                                                    2                    ⁢                    γ                                                        γ                    +                    1                                                  ⁡                                  [                                      1                    -                                          y                                                                        γ                          -                          1                                                γ                                                                              ]                                            }                                                                                      if              ⁢                              xe2x80x83                            ⁢              y                         greater than             0.528                    ⁢                      xe2x80x83                              
and xcex3 is the ratio of specific heats (xcex3=1.4), Pi, Pexh, Pamb are the pressures in the intake manifold, exhaust manifold and at ambient conditions, respectively, Tamb, Texh are the temperatures at ambient conditions and in the exhaust gas, respectively. The parameters uthr and uegr are effective flow areas for the throttle and EGR valve, respectively, as functions of the direct control commands: throttle angle xcex8thr (0 degrees-90 degrees) and the percentage of opening of the EGR valve degr (0-100%). These two functions depend on the geometric configuration of the throttle and EGR valves, respectively, and are identified from the experimental data. In calibrating these two functions, numerical values of uthr and uegr are first calculated from the engine mapping data using equations (1) and (2) for different throttle and EGR valve openings. Then standard regression techniques are applied to correlate uthr with xcex8thr, uegr with degr, respectively.
The standard, open-loop scheme of controlling the cylinder air charge and the burnt gas fraction in the intake system consists of three steps:
1. Given the desired exhaust air-to-fuel ratio rexh,d, the desired in-cylinder flow Wcyl,d and the desired intake manifold burnt gas fraction Fin,d, backtrack the desired EGR flow Wegr,d and throttle flow Wthr,d:                               W                      egr            ,            d                          =                                            F                                                i                  ⁢                                      xe2x80x83                                    ⁢                  n                  ⁢                                      xe2x80x83                                    ⁢                  d                                ⁢                                  xe2x80x83                                                      ⁢                                          W                                  cyl                  ,                  d                                            ⁡                              (                                  1                  +                                      r                                          exh                      ,                      d                                                                      )                                                          1            +                          r              stoich                                                          (        3        )                                          W                      thr            ,            d                          =                              W                          cyl              ,              d                                -                      W                          egr              ,              d                                                          (        4        )            
2. Determine the desired intake manifold pressure Pi,d from the speed density equation (7) below for the given Wcyl,d, "sgr" and N, where the setting "sgr" for the swirl control valve is usually determined from a pre-stored lookup table.
3. Invert the orifice flow representations and the effective flow area functions to determine the desired commands for the throttle and EGR valve effective flow areas                               u                      thr            ,            d                          =                              W                          thr              ,              d                                            f            ⁡                          (                                                                    P                    amb                                                                              T                      amb                                                                      ,                                                      P                                          i                      ,                      d                                                                            P                    amb                                                              )                                                          (        5        )                                          u                      egr            ,            d                          =                              W                          egr              ,              d                                            f            ⁡                          (                                                                    P                    exh                                                                              T                      exh                                                                      ,                                                      P                                          i                      ,                      d                                                                            P                    exh                                                              )                                                          (        6        )            
Then invert the throttle and EGR valve effective flow area functions to determine the desired commands for throttle and EGR valve positions xcex8thr, degr.
The open-loop charge estimation and control approach described above has the advantages of being simple, intuitive and well understood. Its fundamental drawback, however, is the lack of robustness. In particular, this open-loop scheme does not address the following issues that are especially important for DISI engine operation: Limitations and sensitivities of the orifice equation under the high intake manifold pressure conditions (pressure drop close to 1); Lack of on-board measurements for exhaust manifold pressure and temperature; and, Soot deposit buildup and its effects on the engine behavior. Another difficulty in using the orifice equation for flow estimation is its reliance on the knowledge of upstream pressure and temperature. Especially for the EGR valve, the upstream (i.e., exhaust) temperature and pressure vary in a wide range, and no on-board measurement is available for these variables on most production vehicles. Any error in the estimated exhaust temperature and pressure will further deteriorate the quality of flow estimation.
The soot deposit buildup in the intake system or in the EGR conduit is another major problem for a stratified DISI engine. It is largely due to the stratified combustion and high volume of EGR flow. It is very difficult, if not impossible, to predict the effects of the deposits on the effective flow area over time.
In accordance with the present invention, a method is provided for controlling cylinder charge in a direct-injection, spark-ignition engine. The engine includes an intake manifold and an electronically controlled throttle (ETC) valve controlling air flow from the atmosphere to the intake manifold of said engine. The method includes: measuring an intake manifold pressure value, Pi,; measuring a flow value, Wthr,m, which is indicative of flow through the ETC throttle; determining a desired intake manifold pressure value, Pi d; determining a desired ETC valve position, uthr,d; determining a desired flow, Wthr.d, through the ETC valve; and, adjusting said ETC valve position, uthr, in accordance with: said desired ETC valve position, uthr,d; a difference between the measured intake manifold pressure value, Pi, and the determined desired intake manifold pressure value, Pi d; and a difference between the measured flow value, Wthr,m, through the ETC valve and the determined desired flow, Wthr.d.
In accordance with another feature of the invention, a method is provided for controlling cylinder charge in a direct-injection, spark-ignition engine. The engine includes an intake manifold and an electronically controlled throttle (ETC) valve for controlling air flow from the atmosphere to the intake manifold of said engine. The method includes: parameterizing air flow through the ETC valve as:
Wthr=xcex2thr0+xcex2thr1{overscore (W)}thr 
where: xcex2thr0 and xcex2thr 1 are estimator parameters; {overscore (W)}thr is a nominal predicted flow through the ETC valve, such predicted flow being a function of measured ETC valve position and measured intake manifold pressure. Differences between the measured flow through the ETC valve and the predicted flow through the ETC valve are determined. The estimator parameters are modified in accordance with the determined differences. A desired air flow through the ETC valve is provided by adjusting the ETC valve position, uthr, in accordance with the parameterized air flow, such parameterized air flow being a function of the estimator parameters.
In one embodiment, the nominal predicted flow is determined from the standard orifice flow equation.
In one embodiment, the estimator parameters are modified in accordance with:                                                         β              ^                        thr            0                    ⁡                      (                          t              +                              T                a                                      )                          =                                                            β                ^                            thr              0                        ⁡                          (              t              )                                +                                    γ              thr              0                        ⁢                          ϵ              thr                        ⁢                          1                              1                +                                                      W                    _                                    thr                  2                                                                                                                                β              ^                        thr            1                    ⁡                      (                          t              +                              T                a                                      )                          =                                                            β                ^                            thr              1                        ⁡                          (              t              )                                +                                    γ              thr              1                        ⁢                          ϵ              thr                        ⁢                                                            W                  _                                thr                                            1                +                                                      W                    _                                    thr                  2                                                                        
where:
xcex5thr=Wthr,mxe2x88x92Ŵthr; Ŵthr={circumflex over (xcex2)}thr0+{circumflex over (xcex2)}thr 1{overscore (W)}thr and 
xcex3thr0, xcex3thr1, are adaptation gains, t+Ta is a time such adaptation parameters are updated, and Wthr,m is measured flow through the ETC valve.
In one embodiment, the adaptation is disabled when the intake manifold pressure is relatively high and the ETC valve is experiencing a relatively large transient condition.
In one embodiment, the modifying comprises estimating flow through the ETC valve.
In one embodiment, the estimating includes using an adaptation algorithm.
In accordance with another feature of the invention, a method is provided for controlling air charge in a direct-injection, spark-ignition engine. The engine includes an exhaust gas recirculation (EGR) valve connecting the exhaust manifold and intake manifold of the engine and an electronically controlled throttle (ETC) valve controlling air flow from the atmosphere to the intake manifold of said engine. The method includes providing an estimator for estimating flow through the EGR valve. During a calibration mode the method: (a) applies a step function to the estimator, such estimator having a dynamic response characteristic to the applied step function, such dynamic response characteristic being a function of a parameter in such estimator; (b) compares the dynamic response characteristic of the estimator to the step function to the dynamic response characteristic provided in accordance with the orifice equation applied to the intake throttle and the EGR valve; and (c) adjusts the parameter of the estimator to a value where the dynamic response characteristic provided by the estimator is substantially match to a predetermined dynamic response. During a subsequent normal operating mode, the step function is removed and the estimator uses the provided desired value to estimate flow through the EGR valve.
In accordance with another feature of the invention, a method is provided for controlling air charge in a direct-injection, spark-ignition engine. The engine includes an exhaust gas recirculation (EGR) valve connecting the exhaust manifold and intake manifold of the engine and an electronically controlled throttle (ETC) valve controlling air flow from the atmosphere to the intake manifold of said engine. The method includes: (A) measuring an intake manifold pressure value; (B) estimating flow into the cylinder as a function of the measured intake manifold pressure; (C) modifying the estimated flow into the cylinder with a time varying multiplier; (D) determining, when the EGR valve is opened, the time varying multiplier as a function of the difference between measured flow through the ETC and the modified estimated flow into the cylinder with a previously determined multiplier.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.